"I am strong because I eat vegetables."
Translation:Jestem silna dlatego, że jem warzywa.
Hi Amykrumich - you are not alone - see two other similar queries above (including me). I think the refusal of "bo" is simply a system glitch.
But your main point is the difference in use of each form. I don't know! But I dimly remember from somewhere that it is connected to the formality of the situation you are in with "bo" being the more casual and "poniewaz" next. Should still be accepted though.
Fourth time and counting...